Alumina has been used as an envelope material for high intensity alkali metal vapor lamps such as high pressure sodium vapor lamps, because it withstands the attack of the vapors of the alkaline metals even at high temperatures. Yttrium oxide is an excellent candidate material for such high temperature lamp envelopes, and a method of forming transparent or translucent yttria bodies is disclosed in U.S. Pat. No. 4,755,492, incorporated herein by reference. In making such lamps, enclosures which support electrodes in the lamps are bonded and hermetically sealed to the ceramic envelope, for example, by a ceramic sealing composition. The enclosures may take the form of metal end caps which provide direct electrical connection to the electrodes, or ceramic plugs having a metal conductor sealably extending therethrough to provide the electrical connection. Niobium has a coefficient of expansion similar to alumina, and has been used either for the end cap, or the conductor extending through the ceramic plug. U.S. Pat. No. 3,469,729 discloses a sealing composition comprising about 44 to 55 weight percent calcia, about 40 to 55 weight percent alumina, and about 0.5 to 10 weight percent of at least one material selected from the group silica, baria, zirconia, strontia, titania, beryllia, thoria, and yttria. While the conversion from weight percent to mole percent is known, examples of the conversion from weight percent to mole percent of compositions within U.S. Pat. No. 3,469,729 are shown below in Table I.
TABLE I ______________________________________ Conversion of Weight Percent to Mole Percent for Compositions in U.S. Pat. No. 3,469,729 CaO Al.sub.2 O.sub.3 Y.sub.2 O.sub.3 Weight Mole Weight Mole Weight Mole Percent Percent Percent Percent Percent Percent ______________________________________ 44 61.3 46 35.3 10 3.5 44 59.06 55 40.61 1 0.33 46 63.3 44 33.3 10 3.4 50 67.1 40 29.5 10 3.3 45 61 50 37.3 5 1.7 45 61.3 49 36.7 6 2.0 55 70.3 40 28.1 5 1.6 55 69.1 44.5 30.75 0.5 0.2 ______________________________________
One aspect of this invention is to provide a sealing composition suitable for forming seals in high temperature lamps between ceramic members comprised of yttria or alumina, or sealing between refractory metals and the ceramics.